1. Field of the Invention
The present invention relates to a molten carbonate fuel cell and a method of manufacturing a retaining material for an electrolyte body included in the molten carbonate fuel cell, particularly, to a molten carbonate fuel cell comprising an improved electrolyte body interposed between a pair of electrodes made of a conductive material and to a method of manufacturing of improved the retaining material.
2. Description of the Related Art
FIG. 1 shows the basic construction of a molten carbonate fuel cell. As shown in the drawing, the cell comprises an electrolyte body 3 holding an electrolyte consisting of an alkali carbonate. The electrolyte body 3 is held between a pair of conductive electrodes, i.e., an anode 1 and a cathode 2. A fuel gas (H.sub.2, CO.sub.2) is supplied to the anode 1 through a supply port 6 formed in a housing 4a, with the electrolyte body 3 kept molten under high temperatures. Likewise, an oxidizing gas (air, CO.sub.2) is supplied to the cathode 2 through a supply port 8 made in the housing 2. Under these conditions, reactions (1) and (2) given below take place on the surfaces of the anode 1 and cathode 2, respectively, so as to operate the fuel cell: EQU H.sub.2 +CO.sub.3.sup.2- -H.sub.2 O+CO.sub.2 +2e.sup.- (1) EQU 1/2O.sub.2 +CO.sub.2 +2e.sup.- -CO.sub.3.sup.2- (2)
The electrolyte body used in the molten carbonate fuel cell comprises an electrolyte consisting essentially of a mixture of alkali carbonates, a porous body consisting of a retaining material for preventing the electrolyte, which is turned into liquid during operation of the cell at high temperatures, from flowing out of the electrolyte body, and a reinforcing material for preventing the electrolyte body from being cracked when the electrolyte body temperature is elevated. The electrolyte is prepared by mixing at least two of alkali carbonates selected from the group consisting of LiO.sub.2 CO.sub.3, K.sub.2 CO.sub.3 and Na.sub.2 CO.sub.3. A fine powder of .gamma.-lithium aluminate having a particle diameter of 0.05 to 2.0 .mu.m is used as the retaining material. Further, lithium aluminate having a particle diameter of 10 to 100 .mu.m is used as the reinforcing material.
The electrolyte body functions as a medium of movement of carbonate ions (CO.sub.3.sup.2-) and as a barrier layer for preventing reaction gases generated on the surfaces of the anode and cathode from being mixed directly with each other (gas cross-over). To enable the electrolyte body to perform these functions sufficiently, it is necessary for the electrolyte to be retained sufficiently in the electrolyte body. It should be noted that the flow-out of the electrolyte (electrolyte loss) causes not only an increase in the internal resistance of the cell but also generation of the gas cross-over.
It was customary in the past to prepare the electrolyte body by a matrix method in which a porous body having an appropriate fine structure is formed by using the retaining material and the reinforcing material noted above, followed by impregnating the resultant porous body with an electrolyte consisting of a mixture of the alkali carbonates noted above.
In the conventional electrolyte body prepared by the matrix method, however, .gamma.-lithium aluminate used as the retaining material is turned into .alpha.-lithium aluminate during operation of the fuel cell over a long period of time. This change in the phase of lithium aluminate is accompanied by agglomeration of the retaining material particles and by growth of the particles, leading to changes in the fine structure of the porous body (matrix). As a result, coarse pores are generated in the electrolyte body. It follows that an electrolyte loss is brought about, leading to a short life of the fuel cell.
Several measures have been proposed to date in an attempt to overcome the above-noted difficulties inherent in the prior art. For example, Japanese Patent Disclosure (Kokai) No. 63-294668 discloses a technique of using as a retaining material made of lithium aluminate containing as a main component .alpha.-lithium aluminate having an average particle diameter of 0.1 .mu.m. However, the agglomeration or growth of the retaining material particles, which causes changes in the fine structure of the matrix, fails to be prevented even in the case of using an electrolyte body containing .alpha.-lithium aluminate as a main component. It follows that an electrolyte loss is brought about, leading to a short life of the fuel cell, as already pointed out.
Further, Japanese Patent Disclosure No. 2-243511 discloses a technique of carrying out reaction of .gamma.-lithium aluminate having a specific surface area of 20 m.sup.2 /g within a melt to obtain .alpha.-lithium aluminate having a low specific surface area. However, it is necessary to carry out the reaction for at least 5,000 hours for obtaining .alpha.-lithium aluminate of a high crystallinity with a high purity, leading to a high manufacturing cost. Clearly, the particular method is impractical.